netdev - Re: [PATCH net-next 1/3] net: gso: do not include jumbogram HBH header in seglen calculation

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Message-ID: <3728e02b-02d9-4dad-b5da-47e64e91f406@redhat.com>
Date: Tue, 2 Dec 2025 13:06:43 +0100
From: Paolo Abeni <pabeni@...hat.com>
To: Mariusz Klimek <maklimek97@...il.com>, netdev@...r.kernel.org
Subject: Re: [PATCH net-next 1/3] net: gso: do not include jumbogram HBH
 header in seglen calculation

On 12/2/25 12:36 PM, Paolo Abeni wrote:
> On 11/27/25 10:13 AM, Mariusz Klimek wrote:
>> This patch fixes an issue in skb_gso_network_seglen where the calculated
>> segment length includes the HBH headers of BIG TCP jumbograms despite these
>> headers being removed before segmentation. These headers are added by GRO
>> or by ip6_xmit for BIG TCP packets and are later removed by GSO. This bug
>> causes MTU validation of BIG TCP jumbograms to fail.
>>
>> Signed-off-by: Mariusz Klimek <maklimek97@...il.com>
>> ---
>>  net/core/gso.c | 4 ++++
>>  1 file changed, 4 insertions(+)
>>
>> diff --git a/net/core/gso.c b/net/core/gso.c
>> index bcd156372f4d..251a49181031 100644
>> --- a/net/core/gso.c
>> +++ b/net/core/gso.c
>> @@ -180,6 +180,10 @@ static unsigned int skb_gso_network_seglen(const struct sk_buff *skb)
>>  	unsigned int hdr_len = skb_transport_header(skb) -
>>  			       skb_network_header(skb);
>>  
>> +	/* Jumbogram HBH header is removed upon segmentation. */
>> +	if (skb->protocol == htons(ETH_P_IPV6) && skb->len > IPV6_MAXPLEN)
>> +		hdr_len -= sizeof(struct hop_jumbo_hdr);
> 
> Isn't the above condition a bit too course-grain? Specifically, can
> UDP-encapsulated GSO packets wrongly hit it?

I forgot to mention that AI review noted the above check should be
placed in skb_gso_mac_seglen(), too:

https://netdev-ai.bots.linux.dev/ai-review.html?id=bed04a62-0239-4392-a9a3-2399fee27630

AFAICS, it the OVS forwarding path should be impacted.

/P